Various processes have been described for the production of alcohols and diols by hydrogenation of a corresponding unsaturated organic compound selected from esters, diesters and lactones in the presence of a heterogeneous ester hydrogenation catalyst. Such unsaturated organic compounds are unsaturated by virtue of their possessing a carbon-to-oxygen double bond in the linkage --(CO)--O--. They do not need to possess any further unsaturated linkages. Hydrogenation processes of this type are thus applicable to a wide variety of esters, diesters and lactones which contain no unsaturation apart from the afore-mentioned carbon-to-oxygen double bond, for example monoesters of C.sub.8 to C.sub.22 alkylcarboxylic acids, diesters of C.sub.4 to C.sub.16 dicarboxylic acids, and lactones of hydroxycarboxylic acids containing 4 to 16 carbon atoms. However, the presence of further unsaturation in the molecule is not excluded. Thus there can also be used in such processes esters, diesters and lactones which contain further unsaturation in the molecule, for example monoesters of unsaturated C.sub.8 to C.sub.22 aliphatic carboxylic acids, diesters of unsaturated aliphatic or alicyclic carboxylic acids, and unsaturated lactones.
Examples of such hydrogenation processes, many of which are conventionally conducted in the liquid phase, include hydrogenation of alkyl esters of aliphatic monocarboxylic acids to alkanols, and of dialkyl esters of aliphatic dicarboxylic acids to aliphatic diols. It has also been proposed in some cases to effect the hydrogenation reaction under vapour phase reaction conditions.
It is known to produce the cycloaliphatic diol cyclohexanedimethanol by hydrogenation of the corresponding cycloaliphatic diester, usually a dialkyl cyclohexanedicarboxylate, which may itself be produced by hydrogenation of the corresponding dialkyl benzenedicarboxylate, for example dimethyl terephthalate.
A commercial hydrogenation catalyst used for hydrogenation of carboxylic acid esters is copper chromite which may optionally be promoted with barium and/or manganese. The use of such a catalyst in a process for the production of butane-1,4-diol is disclosed in EP-A-0143634. In WO-A-82/03854 there is disclosed a process for effecting the hydrogenolysis of carboxylic acid esters which involves the use of a catalyst comprising a reduced mixture of copper oxide and zinc oxide. Other catalysts useful in hydrogenation reactions which may be mentioned are the palladium/zinc-containing catalysts of WO-A-89/00886 and the mixed catalyst systems of EP-A-0241760. Manganese promoted copper catalysts have also been offered for sale as hydrogenation catalysts.
The hydrogenation reactor or reactors may be operated adiabatically or isothermally with external or internal cooling. Adiabatic reactors are used where possible for preference since they are usually cheaper to construct and to operate than an isothermal reactor of shell and tube design.
The hydrogenation of an ester, diester or lactone feedstock is generally an exothermic reaction. In a liquid phase reaction the feedstock is normally diluted with an inert diluent, conveniently with recycled product hydroxylic compound, and the catalyst is wholly wetted with liquid. The diluent acts as a heat sink and helps to prevent the danger of damage to the catalyst due to the exothermic nature of the hydrogenation reaction.
In a typical vapour phase hydrogenation process the unsaturated organic compound, i.e. the ester, diester or lactone, is normally vaporised directly into a hot hydrogen-containing gas to give a vaporous mixture, which may be heated further or diluted with more hot hydrogen-containing gas in order to raise its temperature above the dew point. It is normally essential to ensure that the vaporous mixture in contact with the catalyst is above its dew point, i.e. above that temperature at which a mixture of gases and vapour just deposits liquid as a fog or a film. This dew point liquid will normally contain all the condensable components of the vapour phase, as well as dissolved gases, in concentrations that satisfy the usual vapour/liquid criteria. It may include the starting material, an intermediate product, a by-product and/or the final hydrogenation product. Generally the process is operated so that the temperature of the vaporous feed mixture is above its dew point, for example about 5.degree. C. to about 10.degree. C. above its dew point. Moreover it is desirable to prevent contact of droplets of liquid with the catalyst, particularly droplets which are rich in the unsaturated organic compound, because damage to the catalyst may result from loss of mechanical strength, from formation of hot spots on the surface of the catalyst or in the pores of the catalyst, due to the exothermic nature of the reaction, leading possibly to sintering and thereby to loss of chemically effective catalyst surface area (particularly in the case of copper-containing catalysts), or from disintegration of the catalyst pellets possibly as a result of explosive vaporisation within the pores of the pellets. Hydrogenation reactor conditions which aim to prevent premature degradation of the hydrogenation catalyst by mechanisms such as the formation of hot spots on the catalyst surface are described in WO-A-91/01961.
Notwithstanding the precautions which may be taken, as described for example in WO-A-91/01961, to maximise the active life of a hydrogenation catalyst, it is still recognised in the art that a hydrogenation catalyst is generally subjected to conditions in the hydrogenation zone which lead inexorably to significant deactivation, and possibly also to irreversible loss of catalytic activity, over a period of time. Such deactivation may be ascribed to different causes in different hydrogenation reactions. For example deposition of carbon or carbonaceous materials on the catalyst surface may be a cause of loss of catalyst activity. In addition to such deactivation processes the catalyst pellets may disintegrate physically in the course of time leading to formation of fines which tend to block the pathway for vapour through the catalyst bed and to lead to an unacceptable increase in pressure drop across the catalyst bed. The deactivation processes may be slowed but not readily reversed.
In any commercial hydrogenation process, the catalyst will eventually lose activity and/or selectivity and need to be replaced with a fresh charge of catalyst. There may be many unrelated but complementary causes of catalyst deactivation in a hydrogenation reaction. These may include i) deposition of carbonaceous materials on the catalyst surface, ii) comminution or structural deterioration of the catalyst pellets resulting from localised physical conditions, iii) poisoning of the catalyst, particularly by compounds containing chlorine or sulphur atoms, and iv) sintering of the catalyst, particularly when the catalyst is a copper-containing catalyst, at high temperatures, for example at temperatures greater than about 230.degree. C.
It is generally recognised in the art that, in hydrogenation reactions utilising copper-containing catalysts, the catalyst is readily deactivated due, it is thought, to sintering or due to migration of metal and is also prone to physical loss of strength such that the catalyst granules tend to disintegrate into a fine powder. Thus it is regarded that catalyst deactivation of copper-containing hydrogenation catalysts is irreversible. Thus, for example, attempts to reactivate copper-containing catalysts which have undergone deactivation as result of prolonged use in the liquid phase hydrogenation of dimethyl 1,4-cyclohexanedicarboxylate according to the teachings of U.S. Pat No. 3,334,149 have not been successful.
Any hydrogenation process which permits reversal or deceleration of deactivation processes is likely to have significant commercial advantages over the processes taught in the prior art due to lower catalyst consumption costs. Such a process would further provide significant environmental benefits resulting from a reduction in catalyst turnover.